Enzymes are known for their ability to selectively catalyze reactions in both aqueous and non-aqueous media. There is a growing interest in the biocatalytic synthesis of specialty polymers since such an approach can generate additional properties such as chirality and biodegradability. Biologically synthesized polymers have been applied as absorbents, biodegradable materials, chiral adsorbents, liquid crystals, and perm-selective membranes. The enzymatic synthesis of oligoesters and polyesters is known and is described, e.g. in Morrow, MRS Bulletin, Nov. 1992, pages 43-47; Geresh et al, Biotechnology and Bioengineering, Vol. 36, 1990, pages 270-274; Morrow et al, Mat. Res. Soc. Symp. Proc., vol. 174, 1990, pages 197-208; Wallace et al, J. Poly. Sci.: Part A: Poly. Chem., Vol 27, 1989, pages 2553-2567 and pages 3271-3284; Margolin et al, Tetrahedron Let., Vol. 28, No. 15, 1987, pages 1607-1610; Binns et al, J. Chem. Soc. Perkin Trans, 1, 1993, pages 899-904; Knani et al, J.Poly. Sci.: Part A: Poly. Chem., Vol 31, 1993, pages 1221-1232 and pages 2887-2897; and, Athawale et al, Biotechnologies Let., Vol. 16, no. 2, Feb., 1994, pages 149-154.
Low molecular weight linear aliphatic oligoesters which are hydroxy terminated have commercial significance for use in the manufacture of polyurethane resins. The current commercial process is based on acid/base catalyzed condensation polymerization between a diacid/diester and a diol. The use of traditional catalysts is limited because such catalysts tend to have an undesirable effect on the subsequent polyurethane synthesis.
Supercritical fluids have been described as extraction solvents and have been used in various industrial extraction processes (see, e.g., Chem. Eng. Sci., Vol. 36, no.11, 1981, pages 1769-1788; and Paulaitis et al, Rev. Chem. Eng., Vol. 1, No. 2, 1983, pages 179-250). Supercritical fluids have also been suggested as being useful for a variety of enzyme catalyzed reactions (see, e.g., Hailing, Enzyme Microb. Tech., March 1994, vol.16, pages 178-206; Randolph et al, Biotech. Let., vol. 7, no. 5, 1985, pages 325-328; Randolph et al, Biocatal. Ind., 1991, Chapter 11, pages 219-237; Aaltonen et al, Chemtech, April 1991, pages 240-248; Perrut, High Pressure and Biotech., 1992, Vol. 224, pages 401-410; Shen et al, Biocatal. in Non-Conventional Media, 1992, pages 417-323; Russell et al, Applied Biochem and Biotech., Vol. 31, 1191, pages 197-211). With supercritical fluids as the reaction medium, enzyme enantioselectivity can be manipulated by the pressure of the system. (see Kamat et al, J. Am. Chem.Soc., 1993, Vol. 115, No. 19, pages 8845-8846). Certain of the work which formed the basis of the present application was described by Russell et al in Chemtech, March 1994, pages 33-37.
In both enzymatic and non-enzyme catalyzed synthesis of oligoesters and polyesters, it is difficult to control product molecular weight in a predictable manner. All techniques described to date depend upon changing the reaction time in order to manipulate molecular weight. In the case of non-enzyme catalyzed oligo- and polyesters, the final product typically contains cyclic ester by-products and small amounts of residual catalyst, both of which can adversely affect properties of products produced from those oligo- and polyesters.